1. Field of the Invention
The present invention relates to a photoelectric transducer that reads an image of an original or the like, and a device and a method for reading an image.
2. Description of the Related Art
An image reading device that reads an image of an original or the like typically includes a charge coupled device (CCD) and an analog front end (AFE) arranged downstream of the CCD. An output of the CCD is input to the AFE via alternating-current (AC)-coupling (via a capacitor connected in series therebetween). Accordingly, only an amount of change (AC) in the output of the CCD is conveyed to the AFE. Put another way, the amount of change in the output of the CCD, or, an output-fluctuation amount, is inevitably conveyed to the AFE. For instance, in a case where an overvoltage above an input voltage rating of the AFE is temporarily generated by the CCD, the overvoltage is directly conveyed to the AFE, causing degradation in characteristics of the AFE device or, at worst, device destruction to occur. Under the circumstance, such an overvoltage input to the AFE has been regarded as an important issue, and there are some known techniques for responding to the issue.
However, the conventional techniques aim at reducing an overvoltage that is developed as a result of at-a-dash emission of unnecessary charges accumulated during power-off of a CCD immediately after power-on of the CCD. Meanwhile, a four-line CCD that generally includes a color (R/G/B) sensor as well as a monochrome (black and white (BW)) sensor has a problem that an overvoltage is developed even in a state where power is kept to be supplied to the CCD. This overvoltage is developed immediately after mode switching between a color reading mode and a monochrome reading mode as a result of at-a-dash emission of unnecessary charges accumulated in one sensor of the sensors that has been at rest before the mode switching. The one sensor the BW sensor when the CCD has been in the color reading mode before the mode switching, while the one sensor is the R/G/B sensor when the CCD has been in the monochrome reading mode.
Meanwhile, when the technique used for switching the mode of the CCD from off state to on state is used for responding to the overvoltage that is developed at switching of the reading mode, it is required to use an additional control signal that is not a control signal for switching of the reading mode. Furthermore, this approach is also disadvantageous in requiring establishing a sequence involving these control signals, thereby making switching of the reading mode time consuming and control complicated.
Japanese Patent Application Laid-open No. 2006-314039 discloses a configuration aimed at reducing magnitude of an influence of an overvoltage input to an AFE. According to the configuration, a clamp circuit unit configured to set an input offset voltage of analog image data to a desired voltage level by charging and discharging an AC coupling capacitor at an input unit of an analog-signal-processing circuit unit. A fixing period over which the clamp unit fixes the input offset voltage to the desired voltage level is set such that the fixing period in a situation where on/off switching of the photoelectric transducer is performed is longer than the fixing period in a situation where the on/off switching is not performed. However, this approach cannot solve the problem that an overvoltage is developed at mode switching between the color reading mode and the monochrome reading mode of the CCD.